A very thorough description of the Ear canal Anatomy is provided in published patent application Ser. No. 10/052,199 to Shennib et al the entirety of said specification is herein incorporated by reference.
Conventional hearing devices are typically characterized by the way they fit into the individual's ear and are:                1) Behind-The-Ear (BTE) type in which the main body consisting of the microphone, power source, amplifier and ear phone/receiver are mounted behind the ear and the sound tube communicating from the earphone to an ear mold which typically fits mostly in the concha or in some instances within the ear canal.        2) In-The-Ear (ITE) type fits largely in the auricle and concha cavity areas, extending minimally into the ear canal. (These are custom fitted)        3) In-The-canal (ITC) type which fits in the concha cavity and extends into the ear canal.        4) Completely-In-the-Canal (CIC) type which fits completely within the ear canal past the aperture.        
Quite recently the “Open-fit” or “Over the Ear” OTE hearing aid have come to the market which are small BTE type hearing aids with a very small delivery sound tube connected to a soft silicone dome or highly vented acrylic tip that holds the tube within the ear canal. These open fit devices are designed to reduce the “occlusion effect”, which is the amplification of your own voice when your ears are blocked. Occlusion effects are annoying in that a users voice sounds unnaturally higher than normal since bone conduction becomes more pronounced as the ear canal is blocked. Teanzer et al. U.S. Pat. No. 6,009,183 and US Patent publication 2005/0190940 to Ach-Kowalewski et al, herein incorporated by reference depict earpieces that mount entirely within the ear canal. U.S. Pat. No. 7,027,608 to Frietz et al herein incorporated by Reference is illustrative of Open fit aids and discusses different ear tips which can be mounted to the sound tube of a BTE hearing aid. The tip of Frietz et al is normally open after insertion into a wearer's ear canal. The eartip of Frietz may be one of several different designs which secures the end of the tube in place in the ear canal in a comfortable manner without the need for an expensive custom made ear mold and without complete occlusion of the ear canal. The eartip of Frietz can have a flower shape, can be dome-shaped with a plurality of vent holes, or can be a foam eartip. The eartip is configured to fit within the ear canal while allowing sounds to outside and within the ear to pass through the ear canal around the eartip.
Nielsen et al, US patent publication 2005/0244026 describes a flexible earpiece for a hearing aid. The flexible earpiece is made of sidewalls which conforms to the wearer's ear canal and attaches to the base of the aid. While Nielsen illustrates that this earpiece connects with the sound tube of a BTE hearing aid, Nielsen do disclose that the earpiece can be used with the base from a hearing aid of the ITC type. The earpiece of Neilson is generally closed in use as the pressure applied to the sidewall by the wearer's ear canal will provide close contact between the overlapping parts of the sidewall so that no leaks occur along the edges of the sidewall. Neilsen et al can allow some air passage through an optional vent at the sidewall base.
US patent publication 2002/0085728 to Shennib et al is descriptive of and extended wear CIC hearing aid wherein the body of the aid is made smaller than a typical ear canal. This design is stated as being mass producible as these do not have to be custom fit to the wearer, as do conventional CIC aids. Shennib minimizes feedback by occluding the bony region with an insert preventing acoustic sound from entering the inner ear.
In general, occlusion in ITC, ITE and CIC aids is somewhat mitigated by a vent tube which provides communication between the ear canal behind the amplified sound source and the surroundings. However the presence of vent tubes or passageways between the amplified sound source and the surroundings leading to unwanted acoustical feedback, which must be carefully managed. Feedback is caused when amplified sound reenters microphone. Therefore, to limit feedback, most CIC devices the vent tube, designed to opening is limited to about 0.6 to 0.8 mm diameter.
U.S. Pat. No. 4,969,534 to Kolpe et al, herein incorporated by reference is illustrative of atypical ITC aid having a casing to which a hollow sleeve is attached which is compressed and inserted in the wears ear. The hollow sleeve allows only amplified sound from the receiver to be transmitted to the tympanic membrane.